Apparatus for measuring pulsed signals using josephson tunneling devices

ABSTRACT

A device and method for measurement of ultra fast waveforms with increased accuracy and storage capability. A superconducting loop contains at least one Josephson tunneling device and is located adjacent to the signal line along which travels the waveform to be measured. Magnetic flux from the waveform intercepts the loop in an amount which is dependent upon the amplitude of the waveform at any instance in time. Control means is provided to switch the Josephson tunneling device to its zero voltage state at the time the waveform is to be sampled. This traps the magnetic flux due to the waveform and the flux will remain stored in a loop as long as the Josephson tunneling device remains in its zero voltage state. A test Josephson tunnel device located adjacent to the superconducting loop can be used to detect the amount of flux trapped in the loop. Non-repetitive pulses can be sampled and the device has a possible resolution of 5 picoseconds. The bandwidth of the non-repetitive signal pulses can be 100 gHz.

SIGNALS USING JOSEPHSON TU NNELING DEVICES [75] Inventor: Hans H. Zappe,Granite Springs,

[73] Assignee: international Business Machines Corporation, Armonk,N.Y..

[22] Filed: June 30, 1972 [21] Appl No.: 267,810

[52] US. Cl 324/102, 307/306,.324/l27,

. 340/173.1 [51] Int. Cl. G0lr 19/00, G0lr 27/28 [58] Field of Search324/102, 127, 43 R,-

Primary Examiner-Robert J. Corcoran Attorney-Jackson E. Stanland et a1.

[57] ABSTRACT A device and method for measurement of ultra fastwaveforms with increased accuracy and storage capability. Asuperconducting loop contains at least one Josephson tunneling deviceand is located adjacent to the signal line along which travels thewaveform to be measured. Magnetic flux from the waveform intercepts theloop in an amount which is dependent upon the amplitude of the waveformat any instance in time. Control means is provided to switch theJosephson tunneling device to its zero voltage state at the time thewaveform is to be sampled. This traps the magnetic flux due to thewaveform and the flux will remain stored in a loop as long as theJosephson tunneling device remains in its zero voltage state. A test.10- sephson tunnel device located adjacent to the superconducting loopcan be used to detect the amount of flux trapped in the loop.Non-repetitive pulses can be sampled and the device has a possibleresolution of 5 picoseconds. The bandwidth of the non-repetitive signalpulses can be 100 gHz.

20 Claims, 11 Drawing Figures [56] References Cited UNITED STATESPATENTS 2,987,631 6/1961 Park, Jr. 307/306 3,259,844 7/1966 Casimir307/306 3,643,237 2/1972 Anacker 307/306 3,705,393 12/1972 Anacker eta1. 307/306 S lGNAL SOURCE SYNC H. MEANS TRIGGER M EANS VOLT METER -58PATENIED UB1 9 I973 14 SIGNAL an SOURCE 220 R0 SYNCH. 18 T MEANS l FJZTRIGGER MEANS I VOLTMETER -58 FIG.'2

CONTROLIC Fm 3 CURRENT 12 11 115; 18 TIME I A PE llrEs I IT l Jfl- LOOPFLUX 42 I I=I f 1 48 J1 J1 1 CURRENT STORED IN J1 A Jm 1 I 1 T T TIME 2TIE i F G. 4 Jm FLUX PATENIEB [JCT 91975 SHEET 2 BF 3 APPARATUS FORMEASURING PULSED SIGNALS USING JOSEPIISON TUNNELING DEVICES BACKGROUNDOF THE INVENTION l. Field of the Invention This invention relates tomethods and apparatus for pulse sampling and analysis, and moreparticularly to a method and apparatus using superconductive loops andJosephson tunneling devices to provide greatly improved results.

2. Description of the Prior Art Pulse measurement and analysis has beenperformed in prior art systems which provide many sampling techniques.An example of a prior art system using magnetic thin films for recordingand storing of a time varying signal is described in U. S. Pat. No.3,656,128. In that reference, an input signal is digitized by a matrixarray and is stored in binary memory cells comprising magnetic thinfilms. These thin films are bistable elements which are switched betweentwo stable states in accordance with the magnetic fields which interceptthem. These fields are due to coincident currents produced in digit andword lines by associated drivers, and in the digit lines by the inputsignal to be recorded.

Prior art pulse sampling and recording means generally cannot be usedfor non-repetitive pulses. That is, for signal pulses which areextremely fast, it has generally been required to have the signal pulserepeat itself so that samplings can be made of it at various times.However, in some types of complex analysis, nonrepetitive signal pulsesmay be created which have to be analyzed and recorded. Prior artsampling means have not been able to do this satisfactorily, with theresult that critical information is often lost.

Prior art sampling means have also not been as fast as would bedesirable, especially in the measurement of ultra fast signal pulses.That is, it is desirable to distinguish changes in an input signal pulsewhich are extremely small, being in the picosecond range.

The prior art pulse sampling techniques have also used generally highimpedance circuitry so that their use has been only in high impedancecircuits. It is desirable to provide a low impedance pulse measurementand recording means which has utility in circuits having very lowimpedance, as for instance, circuits comprising superconductiveelements.

It is also desirable to have a pulse analysis means which can be readout directly without a large amount of associated electronic hardware.That is, each sample point where the signal pulse was recorded should beaccessible directly in order to obtain information about the amplitudeof the signal pulse as a function of time.

The prior art sampling means have not met some or all of these criteria.In addition, elaborate techniques have been devised to record theamplitude of the signal pulses when bistable elements having only twostable states are used in the circuitry. These prior art techniquesutilize matrices of bistable elements which have different thresholds inorder to obtain measurements of different signal amplitudes. However, itis advantageous to avoid the need for storing the signal amplitude invarious locations in a matrix. Also, it is desirable to have a recordingelement which records and stores the exact amplitude of the input signalpulse to be measured regardless of its magnitude, rather than one whichis merely responsive to a single threshold level of amplitude.

Accordingly, it is a primary object of this invention to provide animproved means and method for recording and measuring signal pulses.

It is another object of this invention to provide an improved method andapparatus for recording and measuring input signal pulses which willmeasure these pulses with high resolution.

It is still another object of this invention to provide an improvedmethod and apparatus for recording and measuring signal pulses whichprovides a very close approximation to the exact amplitude of the signalat any instant of time.

It is a further object of this invention to provide improved means formeasurement and recording of signal pulses which will provide indefinitestorage of information representative of the signal pulse.

It is a still further object of this invention to provide an improvedmeans and method for recording and measuring input signal pulses whichhas the capability of directly inducing back into the signal line thepulse shape which was recorded and stored.

It is another object of this invention to provide an improved apparatusfor recording and measurement of input signal pulses which can befabricated in a planar environment and which can be read out directlywith a minimum of associated electronic gear.

It is still another object of this invention to provide an improvedmethod and apparatus for measuring and recording input signal pulseswhich can be used in circuits having very low impedances.

BRIEF SUMMARY OF THE INVENTION The improved method for measurement andrecording of input signal pulses uses the concept of trapping magneticflux due to the signal pulse in a superconductive loop having at leastone Josephson device (such as a tunnel junction or weak link) therein.Magnetic flux from the input signal intercepts the superconductive loopwhen the Josephson tunnel junction is in its resistive state (maximumJosephson current I,,,=O) state. This flux is trapped in asuperconductive loop as a persistent current when the Josephsontunneling device is then switched to its zero voltage state(non'resistive state). The amount of flux trapped in the loop isproportionally quantized to the amplitude of the input signal pulse atthe instant the Josephson tunneling device switches to its zero voltagestate. This closes the loop to trap the flux and provides permanentstorage of the signal pulse, as long as the Josephson tunneling deviceremains in its zero voltage state.

The flux stored in the superconductive loop can be read out directly byusing the persistent current in the loop as a control line for anotherJosephson tunneling device (test device). Depending upon the magnitudeof the persistent current (which in turn depends upon the amount of fluxstored in the loop), the test Josephson jucntion will be switched into anon-zero voltage state which is indicated by a reading on a volt meteror other suitable instrument. The flux in the superconducting loop isread by the test junction bypassing current through the test junctionand noting the magnitude of this current at the time when a voltage isdeveloped across the test junction. This can be directly correlated tothe amplitude of the persistent current in the superconducting loop, tothe flux stored in the superconducting loop, and ultimately to theamplitude of the signal pulse to be measured.

Since a Josephson tunneling device switches rapidly from one voltagestate to the other, a high resolution sampling apparatus is obtained.Further, means are provided to rapidly switch the Josephson tunnelingjunction in the superconductive loop to fully utilize its high speedswitching capabilities.

Any number of superconducting loops having Josephson tunneling junctionstherein can be provided in proximity to the signal line along whichtravels the pulse to be measured. This will provide an indication of thesignal pulse amplitude over a larger distance. In this way, it ispossible to make a definitive measurement of a signal pulse which occursonly once on the signal line.

Because superconducting loops can be directly deposited on a suitablesubstrate, it is possible to use planar fabrication techniques toprovide the entire analytical instrument. Therefore, a measurement andrecording apparatus is provided that is easily fabricated in thin filmform on a substrate.

The flux stored in the superconductive loop will remain storedindefinitely, as long as the Josephson tunneling device(s) in the loopremain in the zero voltage state (superconductive state). Therefore,indefinite storage of the signal is obtained.

Because a superconductive loop having a Josephson tunneling devicetherein can store many flux quanta, a near continuum of steady stateswill be provided. Therefore, it is possible to obtain an exactmeasurement of the amplitude of the input signal. This is in contrastwith sampling apparatus that uses bistable elements which are responsiveto signal amplitudes of a fixed threshold and which provide no furtherinformation even if the amplitude of the signal is significantly greaterthan the threshold amount.

Readout directly by an associated Josephson tunneling device ispossible. This means that minimum electronic gear is required and theentire sampling and readout is performed by devices on the samesubstrate. In addition, the original signal pulse can be re-induced inthe signal line, merely by changing the voltage state of the Josephsontunneling device in the superconductive loop. This will induce a changein flux coupling the signal line which will re-establish the shape ofthe signal pulse in the line.

This invention provides a pulse sampling and recording means using aJosephson tunneling junction in a superconductive loop. In contrast withother sampling scopes, the present invention will work to analyticallyreproduce the signal pulse even though it appears only once on thesignal line. In addition, ultra fast waveforms can be analyzed, whichcould not be achieved with the prior art apparatus.

The present invention recognizes that a structure comprising asuperconducting loop having a Josephson junction therein is not new. Forinstance, U. S. Pat. 3, 363,200 describes the use of a Josephsonjunction device in a superconducting loop in combination with anexternal means for providing a magnetic field intercepting the loop. Thecurrent in the loop is a periodic function of the magnetic fieldintercepting the loop. Also, U. S. Pat. 3,445,760 describes the use of aJosephson tunneling device magnetometer for sensing magnetic fields.

In addition to these references, copending application Ser. No. 51,057,filed June 30, 1970 now U.S. Pat. No. 3,705,393 and assigned to thepresent assignee describes superconducting loops having Josephsonjunctions therein which are used for memory cells. However, the priorart has not recognized that a superior pulse sampling and recordingdevice could be realized with the particular means set forth in thisinvention.

These and other objects, features and advantages will be more fullyapparent from the following more particular description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic illustrationof a pulse analyzing and recording apparatus using Josephson tunnelingjunctions in superconducting loops.

FIG. 2 shows plots of control current and maximum Josephson currentversus time, which are used to explain the apparatus of FIG. 1.

FIG. 3 shows a means for obtaining rapid switching of the Josephsontunneling junction in the measuring loop in order to obtain highresolution measurement and recording.

FIG. 4 shows plots of the various currents in the apparatus of FIG. 3,measured against time.

FIG. 5A shows a schematic illustration of a damped superconducting loopused for measurement and analysis of waveforms, which is capable of highspeed switching between voltage states.

FIGS. 58 and 5C show two embodiments for realizing the circuitry of FIG.5A.

FIG. 6 shows a Josephson tunneling junction having a plurality of seriesconnected junctions, in order to reduce the capacitance of thesuperconducting loop used for measurement and analysis.

FIG. 7 shows a structural diagram of a portion of the apparatus in FIG.1, illustrating the planar method of fabrication of the apparatus ofFIG. 1.

FIG. 8 illustrates the use of plurality of superconductive loops formeasurement of the input signal.

FIG. 9 shows the use of a plurality of connected superconductive loopsfor measurement of the input signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematicillustration of the circuit used to measure a signal pulse representedby waveform l0 traveling in the direction of arrow 12 along signal line14. In this case the signal line is terminated by a resistance R Ofcourse, the signal line can be part of circuitry which is used in someoperation, such as memory or logic, and can be used to provide inputs toother circuitry, rather than being terminated as is shown here.

Signal source 16 provides the waveform 10 to be measured and recorded.This signal source could be a discrete component for providing signals,or could be any other circuitry used to perform another function. Forinstance, signal source 16 could be a complex arrangement of amplifiersand waveform generators used for some other purpose. It may be desirableto analyze the spurious pulses formed from the signal source and theapparatus described herein will adequately do that.

Located adjacent signal line 14 is a superconducting measuring loopgenerally designated 18. Loop 18 comprises a superconductive line 20 andat least one Josephson tunneling device .11. Loop 18 is locatedsufficiently close to signal line 14 that the magnetic field produced bywaveform 10 will intercept loop 18 when waveform 10 travels along signalline 14.

A control means generally designated 22 is used to switch the voltagestate of Josephson tunneling device Jl in a known manner. For instance,control means 22 will provide a control current l which creates amagnetic field intercepting Josephson device Jl. Depending upon themagnitude of the magnetic field intercepting JI, it will be in its zeroresistance state or its finite resistance state. In FIG. 1, controlmeans 22 comprises a current source 24 connected to a conductor 26 whichis sufficiently close to tunnel device J1 that the magnetic fieldestablished by current I, in line 26 will intercept tunnel device J1. Itis not necessary that conductor 26 be comprises of a superconductivematerial, although this is generally the case.

In order to be able to control the voltage state of Josephson tunnelingdevice Jl at the proper time, a synchronization means 28 is responsiveto signal source 16. Synchronization means 28 provides a clock pulse totrigger means 30, which in turn provides a triggerpulse to currentgenerator 24. This controls the magnitude of control current I which inturn controls the voltage state of Josephson tunneling device J1.

As will be apparent when the operation of the circuit of FIG. 1 isdescribed, the magnitude of the current I in superconducting loop 18 ameasure of the amplitude of waveform 10, at the instant of time whenmagnetic flux from waveform is trapped in superconducting loop 18. Inorder to measure current 1,, to therefore determine the amplitude ofwaveform 10 at the instance of triggering, a detection means generallydesignated 32 is provided. Detection means 32 comprises asuperconducting loop 34 having therein another Josephson tunnelingdevice J2. Current I, is provided through tunnel device J2 by currentsource 36. Connected across tunneldevice J2 is a volt meter 38 whichindicates the voltage state of tunnel device J2.

In operation, the current through J2 at which a voltage develops acrossJ2 is dependent upon the magni tude of current I flowing insuperconductive loop 18. Since the magnitude of current I is related tothe amplitude of the waveform 10, the development of voltage on voltmeter 38 occurs at a current I which is indicative of the circulatingcurrent I Operation of the Circuit of FIG. I

In order to explain the operation of the circuit of FIG. I, thewaveforms shown in FIG. 2 will be utilized. These waveforms are plots ofcontrol current I, versus time and maximum Josephson current l throughtunnel device J1, also versus time.

Assume that a complex waveform 10 is traveling in signal line 14 in thedirection of arrow 12. Superconducting loop 18 will be intercepted bythe magnetic flux due to current associated with signal 10. That is,Josephson device J! is in its resistive state at this time so that theflux due to waveform 10 will penetrate superconducting loop 18.

At the time it is desired to record the input signal 10, Josephsondevice Jl is switched to its zero voltage state in order to trap theflux in superconducting loop 18. Device J] is switched to its zerovoltage state by control means 22. Generally, tunnel device Jl willswitch to its zero voltage state when control current I, decreases invalue. This is shown in FIG. 2. That is, when control current I, has itshigher value, the maximum Josephson current l in loop 18 is at a minimum(preferably zero). At that time flux penetrates loop 18. When controlcurrent I decreases to a value I',, the maximum Josephson currentallowed in loop 18 increases and flux will be trapped in the loop atthat time. Accordingly, FIG. 2 shows that at time T1 a sampling of pulse10 is made. Current I, is reduced to a value I, at which time themaximum Josephson current in loop 18 is increased and the flux which haspenetrated the loop is then stored in the loop as a persistent current.Flux will remain stored in loop 18 for as long as control current I, hasthe minimum value I'.. When time T2 is reached, control current Iincreases and the flux stored in the loop will then escape.

The maximum Josephson current possible through device Jl depends on thearea of the tunnel barrier in J], the thickness of the tunnel barrier,etc. That is, it is a function of the Josephson tunneling device itself.However, the maximum amount of flux which is able to penetratesuperconducting loop 18 and be stored therein is a function of themaximum Josephson current through tunnel device J1 and the inductance ofsuperconducting loop 18.

In more detail, the flux intercepting loop 18 is given by the followingexpression:

where A area of loop 18,

n :permeability of the medium inside loop 18,

H(t) the magnetic field intercepting loop 18. Penetration of flux intoloop 18 will occur once the'eddy currents established in this loop diedown. These eddy'currents are produced in opposition to flux penetrationand die out in accordance with the time constant of the superconductingloop 18. Generally, when loop'18 is critically damped, the eddy currentscan die out in approximately 5 picoseconds in a properly designeddevice. Accordingly, the loop can be switched rapidly allowing for avery high resolution sampling apparatus and method.

Flux will be stored in loop 18 in quantized amounts. That is, multiplenumber (N) of flux quanta 41 ((1),, 2 X 10" volt-second) will'betrappedin superconducting loop 18. The maximum amount of flux quanta N whichcan be stored in loop 18 is given by the following expression:

where N l, 2,...,N

L the inductance of loop 18,

I maximum zero voltage current through tunnel device J1.

Current I through loop 18 acts as a control current to determine thevoltage state of test junction J2. That is, the amount of persistentcurrent I flowing in loop 18 will determine the maximum thresholdcurrent I for tunnel device J2. Therefore, tunnel device J2 will beswitched to its finite voltagestate by current in loop 18. This voltagewill occur at a current I which is related to I as explained previously.

Since the amount of persistent current l, in loop 18 is related to theflux from waveform 10 which isin turn related to the amplitude ofwaveform 10, a direct indication will be obtained for the amplitude ofsignal 10.

Generally, it is desirable to use a non-linear Josephson tunnelingdevice (one in which the self-field of the junction is important in itscharacteristic) for device J2. Non-linear Josephson junctions suitablefor this use are described more fully in copending applications Ser. No.158,315 filed June 30, 1971 in the name of H. H. Zappe and Ser. No.194,977 filed Oct. 27, 1971 in the name of J. Matisoo, also assigned tothe present assignee.

Resolution and Bandwidth The bandwidth of an instrument is a measure ofthe resolving power of the instrument in terms of the frequency contentof input signal 10. As the number of frequencies present in input signalincreases, the bandwidth of the instrument must be increased in order toaccurately record and represent the input signal 10.

The resolution of this apparatus is measured by how fast thesuperconducting loop can be switched from a finite voltage state duringwhich flux penetrates the loop to a zero voltage state during which fluxis stored in the loop. This is a function of how quickly eddy currents,set up in loop 18 in opposition to the applied signal, will die down.This in turn is a function of the RLC parameters of superconducting loop18. The resistance of loop 18 is that due to the junction J1 since theloop is superconducting, while the capacitance C of the loop is thatwhich is primarily due to device J1. The inductance of the loop is duealmost entirely to the superconductive line itself.

Generally, maximum resolution is obtained when the RLC circuitrepresenting loop 18 is critically damped. This is easily designeddepending upon the particular junction chosen. Critical damping canprovide a decay time for the eddy currents of approximately 5picoseconds which allows the recording of very rapid changes in inputsignal 10.

Non-Destructive Direct Readout Test Josephson tunnel junction J2provides nondestructive readout of the flux trapped in superconductingloop 18. This is a direct readout since only minimum amounts ofelectronic gear are required to transform the recorded signal. Further,every sample location where the input pulse 10 is recorded is accessibleby the readout meachanism 32.

Reproduction of Input Signal It is possible to re-introduce the shape ofinput signal 10 to signal line 14 after it has been stored insuperconducting loop 18. This is done by changing J1 to its finitevoltage state so that flux stored in loop 18 will leak out and coupleline 14. This will induce a voltage in line 14 which is proportional tothe time derivative of flux previously stored in loop 18.

Measurement of Repetitive Signals It is possible to operate the deviceof FIG. 1 even faster if repetitive signals 10 travel on signal line 14.If tunneling device J1 is switched to its zero voltage state before alleddy currents in superconducting loop 18 have died out, a small amountof flux will be trapped in loop 18. When the pulse 10 reappears,Josephson device J1 is briefly switched to its finite voltage state andthen back to its zero voltage state in order to trap some more fluxtherein. This can be continued until the maximum amount of fluxcorresponding to the signal for that sampling period has been trapped.This may enable faster operation since the problem of eddy current decayis minimized.

Figure 3 FIG. 3 shows a control means for switching the voltage state ofJosephson device J1 rapidly so as to improve resolution of the device ofFIG. 1. In this case, control means 22 comprises a first loop 40 havinga current pulse source 42 therein. Current source 42 produces a pulse ofcurrent 1 through loop 40.

Control means 22 also includes a conductor 44 having a Josephsontunneling device J3 therein. Current I, is provided to Josephson deviceJ3 by the source 46.

Current I in loop 40 is a control current for Josephson device J1, as iscurrent I in conductor 44. Depending upon the presence and absence ofthese currents, the Josephson device Jl will be in either its zerovoltage state or its finite voltage state. Because Josephson device J3is capable of very rapid switching, control means 22 will rapidly switchJ1. This will be more apparent in the following discussion relative toFIG. 4.

Figure 4 FIG. 4 shows plots of the various currents in FIG. 3 in orderto illustrate the operation of control means 22. That is, plottedagainst time are the current values I,, I and I, I

The current I has a direction through portion 48 of loop 40 which isopposite to the direction of current I through conductor 44. This meansthat these currents will have oppositely directed magnetic fieldsintercepting device Jl.

When current I flows and current 1, is not present, device J1 in loop 18has a small maximum Josephson current value (preferably zero). That is,device J1 is in its finite voltage state. During this state, flux entersloop 18. However, when current I flows in conductor 44, the magneticfield established by 1, will be oppositely directed to that establishedby 1,, with the result that the maximum current 1,, which can flowthrough device Jl increases. At this time flux will be stored in thesuperconducting loop 18. As long as current I, flows, device J1 willremain in its zero voltage state.

Figures 5A, 5B and 5C Superconductive loop 18 of FIG. 1 is essentially aparallel R, L, C circuit. For maximum speed of switching from its zerovoltage state to its finite voltage state, superconductive loop 18should be critically damped. The resistance of the loop 18 is theresistance of tunnel device J1, and an external parallel resistor may berequired in order to obtain enough resistance for critical damping. Thatis, the total resistance R of the loop should satisfy the relation R k VL/C for maximum speed.

If the resistance has this value, loop 18 will delay the penetration ofexternal flux with a time constant 7 given by:

erator when the measurement and analysis of signal 2 .pulses is made.Such refrigerators are well known-in the vice J1 is comprised of asuperconductive base elec- Y trode 50, a tunnel barrier 52 and a counterelectrode 54 which is also superconductive. Tunnel barrier 52 isgenerally an oxide of base electrode 50. In this embodiment, small dots56 of a metal which will not oxidize well have been included in tunnelbarrier 52. A suitable example for a metal dot is gold. This increasesthe resistance of tunnel barrier 52 to increase the total loopresistance R.

In FIG. 5C, the same reference numerals are used as were used in FIG.58. That is, base electrode 50 has a tunnel barrier 52 thereon. Locatedover counter electrode 54 is a layer 58 comprised of any normal(nonsuperconducting) metal. A suitable example is gold. It is onlynecessary that layer 58 be not superconducting at the operatingtemperatures of junction J1.

Layer 58 provides a resistive path between base electrode 50 and counterelectrode 54 and thereby serves as the additional resistor R.

Figure 6 In order to reduce the time constant (2RC) of superconductiveloop 18, it is desirable to make the capacitance C as low as possible.Junction capacitances of pF can be realized with this device. Also, thecapcitance may be lowered by replacing the single junction of J1 by astack of tunnel junctions in series to provide a structure in which I isnot affected. This is shown in FIG. 6. Additionally, reference is madeto U. S. Pat. No. 3,643,237 which describes a Josephson tunneling devicecomprising a series of tunnel junctions.

Tunnel device J1 of FIG. 6 comprisesa base electrode 60, a first tunnelbarrier 62, a second electrode 64, a second tunnel barrier 66 and athird electrode 68. Tunnel current I flows between electrodes 60 and 68,passing through the series connected tunnel barriers 62 and 66. Ifdesired, multiple stacks of junctions can be provided in the same tunneldevice, and more than two tunnel junctions can be used in any one stack.

Figure 7 FIG. 7 shows a portion of the structure of the circuit of FIG.1 to illustrate the fabrication of the waveform measuring and analysisdevice. The same reference numerals are used in this FIG. as were usedin FIG. 1.

A superconductive ground plane 70 is provided on which is deposited alayer of insulation 72. The insulation could be, for instance, SiO orany other suitable insulation. Superconductive loop 18 and apparatus 34are provided by depositing superconductive materials onto insulation 72in a known manner. This comprises a series of metal depositions and theprovision of insulating layers between the various layers ofmetallization. In the same manner, insulation is provided over loop 18and conductor 26 of control means 22 is provided over the area ofjunction J1. Insulated from control means 22 is the signal line 14,which is preferably a superconductive material.

The entire'structure of FIG. 7 is very small and can be fabricated in aplanar geometry by known methods. This structure is located in asuitable cryogenic refrigart and will not be described in detail here.

Figure 8 Whereas only one superconductive loop 18 was shown in FIG. 1,it is possible to provide a number of superconducting loops, each ofwhich has at lease one Josephson tunneling device therein. Accordingly,the input signal can then be analyzed throughout its width. This enablesa very accurate representationof the signal to be made even though thesignal is not repetitive. Further, if the superconductive loop controllines are all activated at the same time, a complete waveform can bestored at once which allows sampling and storage of non-repetitivesignals having a bandwidth of the order of gI-Iz. In FIG. 8, a waveform10 to be measured travels in the direction of arrow 12 along signal line14. Instead of using a singlesuperconductive loop 18 as was done in thecircuit of FIG. 1, a plurality of superconducting loops 18-1, 18-2, 18-3and 18-4 are provided. Of course, there can be any number of such loops.The superconducting loops each have a Josephson tunneling devicetherein. For instance, superconductive loop 18-1 has Josephson tunnelingdevice J1 therein, while superconducting loop 18-4 has Josephsontunneling device J4 therein.

Each Josephson tunneling device'Jl, J2, J3, and J4 has associatedtherewith a control means, designated 22-1, 22-2,...22-4, respectively.These control means are generally indicated by a single conductor, thecurrent through which establishes a magnetic field which affects thevoltage state of the Josephson tunneling device in the associated superconductive loop. For instance, control means 22-1 has current I flowingtherein which controls the voltage state of device J1 in loop 18-1.

Associated with each sampling superconductive loop 18-1, etc. is adetection means 32-1, 32-2, 32-3, and 32-4, respectively. Each detectionmeans 32-1, etc. is comprised of a test loop 34-1, 34-2, 34-3 and 34-4,respectively. Each test loop has a Josephson tunneling junction andcurrent meter 35 therein, in the manner shown for test loop 34 ofFIG. 1. For instance, test loop 34-1 has Josephson tunneling device J5therein, while test loop 34-4 has Josephson tunneling device J8 therein.In addition, each test loop has associated therewith a current source36-1, 36-2, 36-3, 36-4 and a voltmeter for determining the voltage stateof the test Josephson device. For instance detection'means 32-4 hascurrent source 36-4 and voltmeter 38-4 for determining the voltage stateof device J8. For ease of drawing, only detection means 32-4 is shown indetail, it being understood that the other test Josephson devices J5, J6and J7 are also provided with current sources and voltmeters for readoutof the voltage state of these Josephson devices. In addition, a decodercan be used to provide the capability of using a common detection meansfor allsuperconductive loops 18.

The circuit of FIG. 8 can be used to determine the amplitude of waveform10 at various places along the waveform in order to reconstruct thewaveform even though it appears only once on line 14. Operation of thecircuitry of FIG. 8 is the same as that of FIG. 1, and will not beexplained further.

Figure 9 FIG. 9 shows another embodiment for utilizing a plurality ofsuperconductive loops 18-1, 18-2, 18-3, and 18-4. Instead of providingseparate superconductive loopsas was done in FIG. 8, the superconductiveloops are connected to one another so that a ladder-type structureresults.

Associated with each of the superconductive test loops 18-1,...18-4 is areadout means 32-1, 32-2, 32-3 and 32-4. For ease of representation,only 32-4 is shown in detail, it being understood that identicalstructures are provided for the other readout means 32-1, 32-2, and32-3. Again, the reference numerals used are the same as those used forFIG. 1 for components performing identical functions.

Operation of the device of FIG. 9 is the same as that of FIG. 8 and FIG.1 and will not be explained further. An advantage of the structure ofFIG. 9 is in fabrication, since each of the superconducting loops isconnected.

What has been described is a device for precise measurement and analysisof complex waveforms which can be of very high frequency, and of anon-repetitive nature. In contrast with the circuitry of the .prior art,very high resolution can be obtained in devices which are small andcompact and which can be fabricated on the same structure as is othercircuitry, such as Josephson memories, logic, etc.

What is claimed is: I

1. An apparatus for measuring pulses of electromagnetic energy,comprising:

a source of said electromagnetic pulses to be measured, said pulsesproducing a magnetic field,

a superconductive loop having a Josephson tunneling device therein whichexhibits a superconducting state and a resistive state, said loop beinglocated in flux-coupling proximity to said magnetic field produced bysaid electromagnetic pulses,

control means for switching said Josephson tunneling 4 device to itssuperconductive state and to its resis- I tive state, there beingpenetration of said loop by said magnetic field when said Josephsontunneling device is in its resistive state, and trapping of saidmagnetic flux when said Josephson tunneling device is switched to itssuperconductive state, detection means for measuring the amount of saidflux trapped in said superconductive loop, said flux being proportionalto the amplitude of said electromagnetic pulse at the time said flux istrapped in said loop.

2. The apparatus of claim 1, including synchronization means coupled tosaid source of electromagnetic pulses and to said control means forproviding clock signals to said control means to trigger the operationof said control means.

3. The apparatus of claim 1, further including additionalsuperconducting loops having Josephson tunneling devices therein, eachof said additional loops being located sufficiently close to said sourceof electromagnetic pulses that the magnetic field produced by saidelectromagnetic pulses magnetically couples to each said additionalloop, each said additional loop having associated therewith a controlmeans for switching the state of the Josephson tunneling device in saidadditionalloop and a detection means for detecting the magnetic fluxstored in said additional loop.

4. The apparatus of claim 1, where said Josephson tunneling device is aplanar device having a planar Josephson tunneling barrier therein.

5. The apparatus of claim I, where said Josephson tunneling device is aweak link. 1

6. The apparatus of claim 1, where said contro means produces a magneticfield which intercepts said Josephson tunneling device.

7. The apparatus of claim 1, where said control means inlcudes at leastone current carrying control line having a control Josephson tunnelingdevice therein having a superconducting state and a resistive state, thestate of said control Josephson tunneling device determining the amountof current flowing in said control line.

8. The apparatus of claim 1, where said detection means inlcudes a testJosephson tunneling device located sufficiently close to saidsuperconducting loop that currents flowing in said loop produce magneticfields which couple said test Josephson tunneling device.

9. The apparatus of claim 1, where the maximum amount N of flux quantastored in said superconducting loop is Ll /tp where L is the inductanceof said superconducting loop and I,, is the maximum Josephson tunnelingcurrent through said Josephson tunneling device. i

10. The apparatus of claim 1, where said superconducting loop iscritically damped.

11. Theapparatus of claim 1, further including a resistor in parallelwith said Josephson tunneling device.

12. The apparatus of claim 1, where said Josephson tunneling device iscomprised of a plurality of Josephson tunneling barriers in series.

13. An-apparatus using Josephson tunneling devices, comprising:

a source of electromagnetic pulses which produce magnetic flux lines,

at least one superconducting loop having a first Josephson tunnelingdevice therein, said loop being magnetically coupled to said flux linesproduced by said electromagnetic pulses, wherein said first Josephsontunneling device has a superconducting state and a resistive state,

control means for switching said first Josephson tunneling devicebetween said superconductive state and said resistive state for trappingin said superconductive loop magnetic flux lines produced by saidelectromagnetic pulses, the amount of flux trapped being proportional tothe amplitude of said electromagnetic pulses at the instant said flux istrapped,

detection means for measuring the amount of flux trapped in saidsuperconducting loop, said detection means including a second Josephsontunneling device magnetically coupled to said superconducting loop.

14. The apparatus of claim 13, further including additionalsuperconducting loops each of which has a Josephson tunneling devicelocated therein, said additional loops being sufficiently close to saidsource of electromagnetic pulses that the magnetic flux lines producedby said electromagnetic pulses couple to said additional loops, eachsaid additional loop having associated therewith a control means forcontrolling the state of said Josephson tunneling device in saidadditional superconducting loop and a detection means for detecting themagnetic fluxstored in said additional su-r perconducting loop.

v 15. The apparatus of claim' 14, wherein each said susource connectedto said second Josephson tunneling device and a current measuring meansconnected to said Josephson tunneling device for measuring the currentthrough said second Josephson tunneling device.

19. The apparatus of claim 14, where said superconductive loops areconnected together.

20. The apparatus of claim 13, where said source of electromagneticpulses includes a current carrying conductor along which saidelectromagnetic pulses travel, said conductor being sufficiently closeto said superconducting loop that magnetic flux produced by saidelectromagnetic pulses couples to said superconducting loop;

i i v

1. An apparatus for measuring pulses of electromagnetic energy,comprising: a source of said electromagnetic pulses to be measured, saidpulses producing a magnetic field, a superconductive loop having aJosephson tunneling device therein which exhibits a superconductingstate and a resistive state, said loop being located in flux-couplingproximity to said magnetic field produced by said electromagneticpulses, control means for switching said Josephson tunneling device toits superconductive state and to its resistive state, there beingpenetration of said loop by said magnetic field when said Josephsontunneling device is in its resistive state, and trapping of saidmagnetic flux when said Josephson tunneling device is switched to itssuperconductive state, detection means for measuring the amount of saidflux trapped in said superconductive loop, said flux being proportionalto the amplitude of said electromagnetic pulse at the time said flux istrapped in said loop.
 2. The apparatus of claim 1, includingsynchronization means coupled to said source of electromagnetic pulsesand to said control means for providing clock signals to said controlmeans to trigger the operation of said control means.
 3. The apparatusof claim 1, further including additional superconducting loops havingJosephson tunneling devices therein, each of said additional loops beinglocated sufficiently close to said source of electromagnetic pulses thatthe magnetic field produced by said electromagnetic pulses magneticallycouples to each said additional loop, each said additional loop havingassociated therewith a control means for switching the state of theJosephson tunneling device in said additional loop and a detection meansfor detecting the magnetic flux stored in said additional loop.
 4. Theapparatus of claim 1, where said Josephson tunneling device is a planardevice having a planar Josephson tunneling barrier therein.
 5. Theapparatus of claim 1, where said Josephson tunneling device is a weaklink.
 6. The apparatus of claim 1, where said control means produces amagnetic field which intercepts said Josephson tunneling device.
 7. Theapparatus of claim 1, where said control means includes at least onecurrent carrying control line having a control Josephson tunnelingdevice therein having a superconducting state and a resistive state, thestate of said control Josephson tunneling device determining the amountof current flowing in said control line.
 8. The apparatus of claim 1,where said detection means inlcudes a test Josephson tunneling devicelocated sufficiently close to said superconducting loop that currentsflowing in said loop produce magnetic fields which couple said testJosephson tunneling device.
 9. The apparatus of claim 1, where themaximum amount N of flux quanta phi 0 stored in said superconductingloop is LIm/ phi 0, where L is the inductance of said superconductingloop and Im is the maximum Josephson tunneling current through saidJosephson tunneling device.
 10. The apparatus of claim 1, where saidsuperconducting loop Is critically damped.
 11. The apparatus of claim 1,further including a resistor in parallel with said Josephson tunnelingdevice.
 12. The apparatus of claim 1, where said Josephson tunnelingdevice is comprised of a plurality of Josephson tunneling barriers inseries.
 13. An apparatus using Josephson tunneling devices, comprising:a source of electromagnetic pulses which produce magnetic flux lines, atleast one superconducting loop having a first Josephson tunneling devicetherein, said loop being magnetically coupled to said flux linesproduced by said electromagnetic pulses, wherein said first Josephsontunneling device has a superconducting state and a resistive state,control means for switching said first Josephson tunneling devicebetween said superconductive state and said resistive state for trappingin said superconductive loop magnetic flux lines produced by saidelectromagnetic pulses, the amount of flux trapped being proportional tothe amplitude of said electromagnetic pulses at the instant said flux istrapped, detection means for measuring the amount of flux trapped insaid superconducting loop, said detection means including a secondJosephson tunneling device magnetically coupled to said superconductingloop.
 14. The apparatus of claim 13, further including additionalsuperconducting loops each of which has a Josephson tunneling devicelocated therein, said additional loops being sufficiently close to saidsource of electromagnetic pulses that the magnetic flux lines producedby said electromagnetic pulses couple to said additional loops, eachsaid additional loop having associated therewith a control means forcontrolling the state of said Josephson tunneling device in saidadditional superconducting loop and a detection means for detecting themagnetic flux stored in said additional superconducting loop.
 15. Theapparatus of claim 14, wherein each said superconducting loop iscritically damped.
 16. The apparatus of claim 13, where said controlmeans includes at least one current carrying control line having acontrol Josephson tunneling device exhibiting a superconducting stateand a resistive state, the state of said control Josephson tunnelingdevice determining the amount of current flowing in said control line.17. The apparatus of claim 13, where said superconducting loop iscritically damped.
 18. The apparatus of claim 13, including a currentsource connected to said second Josephson tunneling device and a currentmeasuring means connected to said Josephson tunneling device formeasuring the current through said second Josephson tunneling device.19. The apparatus of claim 14, where said superconductive loops areconnected together.
 20. The apparatus of claim 13, where said source ofelectromagnetic pulses includes a current carrying conductor along whichsaid electromagnetic pulses travel, said conductor being sufficientlyclose to said superconducting loop that magnetic flux produced by saidelectromagnetic pulses couples to said superconducting loop.